This invention relates to a method for determining the mass flow rate by the Coriolis principle whereby, with the aid of at least two process-variable detectors, at least two data signals reflecting the movements of a Coriolis line are generated, the said data signals are processed with the aid of at least three parallel input stages, each stage having an input contact, the phase differences between the processed data signals are determined with the aid of at least two phase detectors, the data signals are, at least in part, fed to the input contacts of the input stages with the aid of at least one first check switch, a correcting circuit derives from the phase differences between the processed data signals a corrected phase difference between these data signals, and with the aid of an evaluation unit, the mass-flow rate is established on the basis of the corrected phase difference.
The method used for measuring the phase difference between two data signals, constituting the underlying concept of the procedure according to this invention for determining the mass flow rate according to the Coriolis principle, can also be employed in connection with other measuring processes where it is important to define the phase difference between two data signals.
The fundamental objective of this invention is an improved signal evaluation method for Coriolis-type mass flowmeters. The data signals provided by these Coriolis mass flowmeters are in the form of two periodic voltages with a phase shift that is proportional to the mass flow rate. The accuracy of the mass flowmeters is thus a function of the determination of the phase difference. For accuracy requirements common in the industry, the resolution of the phase difference must be approximately 4xc3x9710xe2x88x925 degrees and these requirements are becoming increasingly stringent. A major obstacle in meeting these requirements is the temperature-dependent phase shift encountered in the analog part of the evaluation circuitry, and especially in the input stage, but also in the leads between the process-variable detectors and the input stages. U.S. Pat. No. 4,817,448 describes an example of a method for correcting these phase shifts. That method employs a polarity reversal circuit for periodically switching the polarity of the data signals between the input states. Then, if temperature changes are not a factor, the phase detector supplies precisely inverted values respectively. However, if the input stages are exposed to temperature influences respectively, there will indeed be different phase shifts, as a result of which the phase-difference values supplied by the phase detector upon polarity reversal are no longer precisely inverted relative to each other. From this phase-difference deviation a zeroing signal is derived which then serves to correct the phase differences detected, thus meeting the accuracy requirements in measuring the phase difference.
The problem with this earlier method for determining the mass flow rate by the Coriolis principle is that polarity reversal produces transients which prohibit the evaluation of the phase detector readings for a fairly significant length of time. This is particularly undesirable when the mass flow data are used for controlling feedback loops, since in that case, it is impossible at certain time intervals to determine the controlled variable. After the transient which appeared during the transitional period in the determination of the phase difference have abated, this earlier method provides for the second data signal to be switched to the input contact of the remaining input stage so that the phase detector which up to that point had transmitted the uncorrected phase difference between the two data signals, will now measure the difference between the temperature-dependent phase shift in the first-mentioned input stage and the third input stage. Since in that earlier method, addressed by this invention, the phase-shift differences between the first and third input stages and the second and third input stages are measured, the stored phase-shift differences concerned can be used for correcting the uncorrected phase difference between the first and the second data signal present at the first or, respectively, second phase detector.
Another example of a method for determining the mass-flow rate according to the Coriolis principle, on which this invention is based, is described in EP 0 593 623 B1. According to that earlier method, first and second input signals are alternated, by way of a first check switch, between the input contacts of first and the second input stages while one of the two data signals remains continuously connected to the input contact of a third input stage. In an implementation of this earlier method, the processed data signals of the first input stage and the third input stage are continuously fed to a first phase detector while the processed data signals of the second input stage and the third input stage are continuously fed to a second phase detector. If, for example, the second data signal is continuously connected to the input contact of the third input stage, one of the input contacts of the first or second input stage will always have to receive the first data signal. In this earlier concept, the input contact of the remaining input stage temporarily receives the second data signal, so that the second data signal processed by the third input stage and the data signal processed by the remaining input stage are fed to one of the phase detectors, making it possible with the aid of this phase detector to determine the difference in the temperature dependent phase shifts in the input stages concerned. The remaining phase detector receives the first data signal processed by an input stage as well as the second data signal processed by the third input stage, so that this phase detector measures the uncorrected phase difference between the first and the second data signals. At this point in the approach described, the first data signal is also fed to the input contact of the remaining input stage so that, during a transitional period, both phase detectors transmit the uncorrected phase difference between the two data signals. After the transients which appeared during the transitional period in the determination of the phase difference have abated, this earlier method provides for the second data signal to be switched to the input contact of the remaining input stage so that the phase detector which up to that point had transmitted the uncorrected phase difference between the two data signals, will now measure the difference between the temperature-dependent phase shift in the first-mentioned input stage and the third input stage. Since in that earlier method, addressed by this invention, the phase-shift differences between the first and third input stages and the second and third input stages are measured, the stored phase-shift differences concerned can be used for correcting the uncorrected phase difference between the first and the second data signals present at the first or, respectively, second phase detector.
This means that the earlier method, to which this invention refers, produces a measurement of the phase difference between two data signals which has been corrected for the temperature dependent phase shifts induced in the input stages. At the same time, the data signal for the phase difference is continually available because, when the data signals are switched from one input stage to another input stage by means of the check switch, the transients produced in the process can be allowed to abate before using the output signal of the other phase detector instead of the output signal of the one phase detector as the reading for the uncorrected phase difference between the first and the second data signals.
The problem with the earlier method for determining the mass-flow rate by the Coriolis principle, on which this invention is based, is that the data signals are fed to the input contacts of the input stages by way of a relatively complicated check switch in what is a particularly interference-prone section preceding the input stages. Cross-talk between the data signals is therefore not altogether avoidable. Moreover, the earlier method does not allow for a design adaptation of the phase detectors to varying requirements since both phase detectors are employed in alternating fashion for measuring the uncorrected phase difference and determining a corrective phase difference.
This invention is, therefore, aimed at improving the earlier method for determining the mass flow rate based on the Coriolis principle, in such a manner as to provide greater measuring accuracy while reducing the complexity of the electronics.
According to this invention, the inherent problem indicated above is solved with a design whereby the first phase detector serves to continuously measure the uncorrected phase difference between the first data signal processed with the aid of the first input stage and the second data signal processed with the aid of the second input stage, while at least periodically measuring a first corrective phase difference between the first data signal processed in the first input stage and the first data signal processed in the third input stage and, in alternating fashion, a second corrective phase difference between the second data signal processed with the aid of the second input stage and the second data signal processed in the third input stage, and the correction circuitry serves to determine the corrected phase difference between the data signals derived from the sum of the uncorrected phase difference and the difference between the first and the second corrective phase differences.
This invention ensures that, prior to reaching the input stages, the data signals only need to be alternately fed to the input contact of the third input stage by means of a simple check switch. This simplifies the electronics otherwise needed for measuring accuracy in the particularly sensitive area in front of the input stages. Moreover in the design according to this invention, the uncorrected phase difference is determined by the first phase detector, and the corrective phase differences by at least one second phase detector. Accordingly, these phase detectors can be optimally adapted to any given requirements.
In a particularly simple implementation of the method according to this invention, a second check switch shunts the processed data signals to the input contacts of the second phase detector and the second phase detector serves to measure the first and the second corrective phase difference. Alternatively it would be possible, albeit more expensively, to use the second phase detector exclusively for determining the first corrective phase difference while employing a third phase detector for determining the second corrective phase difference.
If prior to the measurement of the phase differences the processed data signals are digitized, any additional influence by temperature susceptible components is minimized since the digital measurement of the phase differences is totally unaffected by temperature influences.
Of course, if prior to the measurement of the phase differences the processed data signals are to be digitized, the phase detectors must be of the digital type. In this context, one may want to employ Fourier transform-processed data signals for measuring the phase differences as described for instance in EP 0 282 552 B1.
The processed data signals can be sampled in synchronous or asynchronous fashion. Asynchronous sampling offers the advantage of permitting the use of fewer analog/digital converters which, of course, requires faster analog/digital converters.
Since the effect of temperature variations on the phase shifts in the input stages displays a relatively large time constant, it provides the opportunity to periodically interrupt the corrected phase difference measurements, having the ability during these interruptions, without any loss of accuracy, to utilize the stored corrective phase difference values for correction purpose while the freed-up capacities of the analog/digital converters are available for digitizing other variables, for instance the temperatures of the Coriolis line.
There are various ways in which the procedural concept according to this invention for determining the mass flow rate according to the Coriolis principle can be designed and enhanced. In this connection, reference is made to the dependent claims, and to the description and drawing of a preferred implementation example.